The amount of emissions, such as NOx, discharged from an internal combustion engine such as a spark-ignition internal combustion engine or a diesel engine has a strong correlation with the flame temperature (combustion temperature) after ignition. Therefore, controlling the flame temperature to a predetermined temperature effectively reduces the amount of emissions, such as NOx. In general, since flame temperature cannot be detected directly, the flame temperature must be estimated so as to control the flame temperature to the predetermined temperature. Meanwhile, the flame temperature changes depending on the temperature within a cylinder before ignition (hereinafter, may be simply referred to as “cylinder interior temperature”). Accordingly, estimating the cylinder interior temperature is effective for estimation of the flame temperature.
In particular, in the case of a diesel engine in which air-fuel mixture starts combustion by means of self ignition caused by compression, the ignition timing must be properly controlled in accordance with the operating state of the engine. The ignition timing greatly depends on the cylinder interior temperature before ignition. Accordingly, estimation of the cylinder interior temperature is also necessary for proper control of the ignition timing.
In view of the above, a fuel injection apparatus for a diesel engine disclosed in Japanese Patent Application Laid-Open (kokai) No. 2001-254645 sets a target ignition timing in accordance with the operation state of an engine, and estimates a cylinder interior temperature as measured at the target ignition timing on the basis of various operational state quantities which affect the cylinder interior temperature, such as engine coolant temperature, intake air temperature, and intake pressure. Subsequently, the apparatus controls the manner of injection (e.g., injection timing and/or injection pressure) of fuel in such a manner that the estimated cylinder interior temperature attains a predetermined temperature, to thereby control the ignition timing to coincide with the target ignition timing.
Incidentally, strictly speaking, the above-mentioned flame temperature and, therefore, the timing of ignition (by means of self ignition) changes depending on the pre-ignition temperature of a gas mixture produced through mixing of fuel (fuel vapor) present within a cylinder and a gas (new air, EGR gas, etc.) present within the cylinder (hereinafter, the pre-ignition temperature will be referred to as simply the “temperature of gas mixture”).
In particular, in the case of an internal combustion engine of a type in which fuel is injected directly to each cylinder, the injected fuel is ignited before it is uniformly mixed with the entirety of gas already taken into the cylinder (hereinafter referred to as “cylinder interior gas”). In other words, at or immediately before an ignition timing, the gas mixture consists of the injected fuel (fuel vapor) and a portion of the cylinder interior gas. Accordingly, at the ignition timing, in the cylinder, the gas mixture occupies one spatial region, and the cylinder interior gas which is present around the gas mixture without being mixed with the fuel (hereinafter may be referred to as “peripheral internal combustion gas”) occupies a different spatial region. Since the temperature at the region occupied by the gas mixture (accordingly, the temperature of the gas mixture) naturally differs from that of the peripheral internal combustion gas, the temperature distribution within the cylinder is not uniform.
However, the conventional apparatus estimates the cylinder interior temperature under the assumption that fuel (fuel vapor) is uniformly mixed with the entirety of cylinder interior gas and that a resultant gas mixture is uniformly present within the entire cylinder. Accordingly, the estimated cylinder interior temperature differs from the temperature of the gas mixture itself. As a result, the apparatus fails to accurately control the ignition timing to the target ignition timing. In view of the above, there has arisen desire to accurately estimate the temperature (state quantity) of a gas mixture consisting of injected fuel (fuel vapor) and a portion of cylinder interior gas mixed with the fuel.